Bulletin 2184-E

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Abstract

The potential of Lower Cretaceous sandstones of the
Travis Peak Formation in the northern Gulf Coast Basin to
harbor a basin-centered gas accumulation was evaluated by
examining (1) the depositional and diagenetic history and reservoir
properties of Travis Peak sandstones, (2) the presence
and quality of source rocks for generating gas, (3) the burial
and thermal history of source rocks and time of gas generation
and migration relative to tectonic development of Travis Peak
traps, (4) gas and water recoveries from drill-stem and formation
tests, (5) the distribution of abnormal pressures based
on shut-in-pressure data, and (6) the presence or absence of
gas-water contacts associated with gas accumulations in Travis
Peak sandstones.
The Travis Peak Formation (and correlative Hosston Formation)
is a basinward-thickening wedge of terrigenous clastic
sedimentary rocks that underlies the northern Gulf Coast Basin
from eastern Texas across northern Louisiana to southern Mississippi.
Clastic infl ux was focused in two main fl uvial-deltaic
depocenters?one located in northeastern Texas and the other
in southeastern Mississippi and northeastern Louisiana. Across
the main hydrocarbon-productive trend in eastern Texas and
northern Louisiana, the Travis Peak Formation is about 2,000
ft thick.
Most Travis Peak hydrocarbon production in eastern
Texas comes from drilling depths between 6,000 and 10,000
ft. Signifi cant decrease in porosity and permeability occurs
through that depth interval. Above 8,000-ft drilling depth in
eastern Texas, Travis Peak sandstone matrix permeabilities
often are signifi cantly higher than the 0.1-millidarcy (mD)
cutoff that characterizes tight-gas reservoirs. Below 8,000 ft,
matrix permeability of Travis Peak sandstones is low because
of pervasive quartz cementation, but abundant natural fractures
impart signifi cant fracture permeability.
Although pressure data within the middle and lower
Travis Peak Formation are limited in eastern Texas, overpressured
reservoirs caused by thermal generation of gas, typical
of basin-centered gas accumulations, are not common in the
Travis Peak Formation. Signifi cant overpressure was found in
only one Travis Peak sandstone reservoir in 1 of 24 oil and gas
fi elds examined across eastern Texas and northern Louisiana.
The presence of gas-water contacts is perhaps the most
defi nitive criterion indicating that a gas accumulation is conventional
rather than a ?sweet spot? within a basin-centered
gas accumulation. Hydrocarbon-water contacts within Travis
Peak sandstone reservoirs were documented in 17 fi elds and
probably occur in considerably more fi elds across the productive
Travis Peak trend in eastern Texas and northern Louisiana.
All known hydrocarbon-water contacts in Travis Peak reservoirs
in eastern Texas, however, occur within sandstones in the
upper 500 ft of the formation. Although no gas-water contacts
have been reported within the lower three-fourths of the Travis
Peak Formation in northeastern Texas, gas production from
that interval is limited. The best available data suggest that
most middle and lower Travis Peak sandstones are water bearing
in northeastern Texas.
Insuffi cient hydrocarbon charge relative to permeability
of Travis Peak reservoirs might be responsible for lack of
overpressure and basin-centered gas within the Travis Peak
Formation. Shales interbedded with Travis Peak sandstones in
eastern Texas are primarily oxidized fl ood-plain deposits with
insuffi cient organic-carbon content to be signifi cant sources
of oil and gas. The most likely source rocks for hydrocarbons
in Travis Peak reservoirs are two stratigraphically lower units,
the Jurassic-age Bossier Shale of the Cotton Valley Group,
and laminated, lime mudstones of the Jurassic Smackover
Formation. Hydrocarbon charge, therefore, might be suffi cient
for development of conventional gas accumulations, but it is
insuffi cient for